In specific binding assays, sensitivity is of prime importance due to the generally low analyte levels that are measured. Radioimmunoassay sensitivity limits the assay to measurements of concentration of 10.sup.-12 M, and more often only in the 10.sup.-8 to 10.sup.-10 M range. In addition, radiolabels suffer from the drawbacks of short half-life and handling hazards.
In fluorescence spectroscopy assays, a sample containing a fluorescent species to be analyzed is irradiated with light of known spectral distribution within the excitation spectrum of the target fluorescent species. The intensity of the resulting characteristic emission spectrum of the fluorescent target molecules is determined and is related to the number of target molecules.
The sensitivity of fluorescence assays, although theoretically very high, is limited by the presence of background fluorescence. Background signal levels are picked up from competing fluorescent substances, not only in the sample, but also in materials containing the sample. This is an especially serious problem in quantitative measurements of species associated with the samples containing low concentrations of desired target fluorescent molecules such as found in biological fluids. In many situations, it is impossible to reduce the background sufficiently (by appropriate filtration and other techniques known in the art) to obtain the desired sensitivity.
Time resolution offers an independent means of isolating the specific fluorescent signal of interest from nonspecific background fluorescence. Time resolution is possible if the label has much longer-lived fluorescence than the background, and if the system is illuminated by an intermittent light source such that the long-lived label is measurable during the dark period subsequent to the decay of the short-lived background. Such techniques are described in greater detail in German Offenlegungsschrift No. 2,628,158 published Dec. 30, 1976.
The long-lived fluorescence (0.1-5 msec) of the aromatic diketone chelates of certain rare-earth metals, for example, europiumbenzoylacetonate and europiumbenzoyl-trifluoracetonate, has been known for some time. The chelating agent absorbs light and transfers it to the metal ion, which fluoresces. German OLS No. 2,628,158 describes the use of time resolution in fluorometric immunoassays (FIA) through the use of fluorescent labels whose emissions are long-lived as compared with those of species which produce background interferences in such assays. This publication also provides a useful discussion of the techniques of FIA and its advantage over other immunoassay techniques such as radioimmunoassay (RIA).
The fluorescent immunoreagents described in German OLS No. 2,268,158 comprise at least one member of the immune system, i.e. an antibody or an antigen, "conjugated" with a rare-earth chelate. Such "conjugation" can be achieved in one of two ways:
(1) by labeling, i.e. attaching the rare-earth chelate to the antigen as described in Fluorescent Antibody Techniques and Their Application by A. Kawamura, Ed., University Park Press, Baltimore, Maryland, 1969, and then adding antibody to the conjugated antigen whereby the antibody and antigen join in the usual fashion, or
(2) by covalent bonding of the antibody of the chelate via a chemical group which binds to both antibodies and the chelates.
The problem with immunoreagents of the type described in German OLS No. 2,268,158 is that the fluorescent labeling species, namely the rare-earth chelates, are quenched, i.e. their fluorescence is extinguished, when contacted with water. This problem, hereinafter referred to as an "aqueous stability" problem, is particularly serious because a principal use for fluorescent labeled immunoreagents is in the assay of aqueous biological liquids such as blood, serum, etc. If aqueous stability could be conferred on these materials, they would be useful as fluorescent labels, for these biological liquids, thus allowing increased fluorescence immunoassay sensitivity by the use of time resolution of signal from background.
Further rare-earth chelates previously used for fluorometric measurements have had undesirable properties such as a low quantum yield for emission, undesirable sensitizer extinction coefficients which result in insufficient fluorescence using small quantities of detectable species, low .lambda.max which renders the determination subject to interference from other components in the sample which are usually in the low .lambda.max range, poor water solubility (most biological fluids are aqueous) and poor stability of the chelate at low concentrations.